1. Field of the Invention
This invention relates to electronic systems, and more particularly to power distribution networks embodied within printed circuit boards and semiconductor device packages having continuous planar conductors.
2. Description of the Related Art
Electronic systems typically employ several different types of electrical interconnecting apparatus having planar layers of electrically conductive material (i.e., planar conductors) separated by dielectric layers. A portion of the conductive layers may be patterned to form electrically conductive signal lines or xe2x80x9ctracesxe2x80x9d. Conductive traces in different layers (i.e., on different levels) are typically connected using contact structures formed in openings in the dielectric layers (i.e., vias). For example, printed circuit boards typically have several layers of conductive traces separated by dielectric layers. The conductive traces are used to electrically interconnect terminals of electronic devices mounted upon the PCB. Similarly, semiconductor device packages often have several layers of conductive traces separated by dielectric layers to electronically connect bonding pads of an integrated circuit to terminals (e.g., pins or leads) of the device package.
Signals in digital electronic systems typically carry information by alternating between two voltage levels (i.e., a low voltage level and a high voltage level). A digital signal cannot transition instantaneously from the low voltage level to the high voltage level, or vice versa. The finite amount of time during which a digital signal transitions from the low voltage level to the high voltage level is called the rise time of the signal. Similarly, the finite amount of time during which a digital signal transitions from the high voltage level to the low voltage level is called the fall time of the signal.
Digital electronic systems are continually being produced which operate at higher signal frequencies (i.e., higher speeds). In order for the digital signals within such systems to remain stable for appreciable periods of time between transitions, the rise and fall times of the signals must decrease as signal frequencies increase. This decrease in signal transition times (i.e., rise and fall times) creates several problems within digital electronic systems, including signal degradation due to reflections, power supply xe2x80x9cdroopxe2x80x9d, ground xe2x80x9cbouncexe2x80x9d, and increased electromagnetic emissions.
A signal driven upon (i.e., launched) from a source end of a conductive trace suffers degradation when a portion of the signal reflected from a load end of the trace arrives at the source end after the transition is complete (i.e., after the rise time or fall time of the signal). A portion of the signal is reflected back from the load end of the trace when the input impedance of the load does not match the characteristic impedance of the trace. When the length of a conductive trace is greater than the signal transition time (i.e., the rise or fall time) divided by about 20 times the delay per unit length along the trace, the effects of reflections upon signal integrity (i.e., transmission line effects) should be considered. If necessary, steps should be taken to minimize the degradations of signals conveyed upon the trace due to reflections. The act of altering impedances at the source or load ends of the trace in order to reduce signal reflections is referred to as xe2x80x9cterminatingxe2x80x9d the trace. For example, the input impedance of the load may be altered to match the characteristic impedance of the trace in order to prevent signal reflection. As the transition time (i.e., the rise or fall time) of the signal decreases, so does the length of trace which must be terminated in order to reduce signal degradation.
A digital signal alternating between the high and low voltage levels includes contributions from a fundamental sinusoidal frequency (i.e., a first harmonic) and integer multiples of the first harmonic. As the rise and fall times of a digital signal decrease, the magnitudes of a greater number of the integer multiples of the first harmonic become significant. As a general rule, the frequency content of a digital signal extends to a frequency equal to the reciprocal of xcfx80 times the transition time (i.e., rise or fall time) of the signal. For example, a digital signal with a 1 nanosecond transition time has a frequency content extending up to about 318 MHz.
All conductors have a certain amount of inductance. The voltage across the inductance of a conductor is directly proportional to the rate of change of current through the conductor. At the high frequencies present in conductors carrying digital signals having short transition times, a significant voltage drop occurs across a conductor having even a small inductance. A power supply conductor connects one terminal of an electrical power supply to a power supply terminal of a device, and a ground conductor connects a ground terminal of the power supply to a ground terminal of the device. When the device generates a digital signal having short transition times, high frequency transient load currents flow in the power supply and ground conductors. Power supply droop is the term used to describe the decrease in voltage at the power supply terminal of the device due to the flow of transient load current through the inductance of the power supply conductor. Similarly, ground bounce is the term used to describe the increase in voltage at the ground terminal of the device due to the flow of transient load current through the inductance of the ground conductor. When the device generates several digital signals having short transition times simultaneously, the power supply droop and ground bounce effects are additive. Sufficient power supply droop and ground bounce can cause the device to fail to function correctly.
Power supply droop is commonly reduced by arranging power supply conductors to form a crisscross network of intersecting power supply conductors (i.e., a power supply grid). Such a grid network has a lower inductance, hence power supply droop is reduced. A continuous power supply plane may also be provided which has an even lower inductance than a grid network. Placing a xe2x80x9cbypassxe2x80x9d capacitor near the power supply terminal of the device is also used to reduce power supply droop. The bypass capacitor supplies a substantial amount of the transient load current, thereby reducing the amount of transient load current flowing through the power supply conductor. Ground bounce is reduced by using a low inductance ground conductor grid network, or a continuous ground plane having an even lower amount of inductance. Power supply and ground grids or planes are commonly placed in close proximity to one another in order to further reduce the inductances of the grids or planes.
Electromagnetic interference (EMI) is the term used to describe unwanted interference energies either conducted as currents or radiated as electromagnetic fields. High frequency components present within circuits producing digital signals having short transition times may be coupled into nearby electronic systems (e.g., radio and television circuits), disrupting proper operation of these systems. The United States Federal Communication Commission has established upper limits for the amounts of EMI products for sale in the United States may generate.
Signal circuits form current loops which radiate magnetic fields in a differential mode. Differential mode EMI is usually reduced by reducing the areas proscribed by the circuits and the magnitudes of the signal currents. Impedances of power and ground conductors create voltage drops along the conductors, causing the conductors to radiate electric fields in a common mode. Common mode EMI is typically reduced by reducing the impedances of the power and ground conductors. Reducing the impedances of the power and ground conductors thus reduces EMI as well as power supply droop and ground bounce.
Within the wide frequency range present within electronic systems with digital signals having short transition times, the electrical impedance between any two parallel conductive planes (e.g., adjacent power and ground planes) may vary widely. The parallel conductive planes may exhibit multiple electrical resonances, resulting in alternating high and low impedance values. Parallel conductive planes tend to radiate a significant amount of differential mode EMI at their boundaries (i.e., from their edges). The magnitude of differential mode EMI radiated from the edges of the parallel conductive planes varies with frequency and is directly proportional to the electrical impedance between the planes.
FIG. 1 is a perspective view of a pair of 10 in.xc3x9710 in. square conductive planes separated by a fiberglass-epoxy composite dielectric layer. Each conductive plane is made of copper and is 0.0014 in. (1.4 mils) thick. The fiberglass-epoxy composite layer separating the planes has a dielectric constant of 4.0 and is 0.004 in. (4 mils) thick. FIG. 2 is a graph of the magnitude of the simulated electrical impedance between the pair of rectangular conductive planes of FIG. 1 (log10 scale) versus the frequency of a voltage between the planes (log10 scale). The graph was created by modeling each square inch of the pair of conductive planes as a matrix of transmission line segments. The impedance value was computed by simulating the application of a 1 ampere constant current between the centers of the rectangular planes, varying the frequency of the current, and determining the magnitude of the steady state voltage between the centers of the rectangular planes.
As shown in FIG. 2, the magnitude of the electrical impedance between the parallel conductive planes of FIG. 1 varies widely at frequencies above about 20 MHz. The parallel conductive planes exhibit multiple electrical resonances at frequencies between 100 MHz and 1 GHz, resulting in alternating high and low impedance values. The parallel conductive planes of FIG. 1 tend to radiate substantial amounts of EMI at frequencies where the electrical impedance between the planes anywhere near their peripheries is high.
It would thus be desirable to provide a power distribution network wherein the electrical impedance between parallel conductive planes may be stabilized. Such a network would reduce power supply droop, ground bounce, and the amount of electromagnetic energy radiated from the edges of the planes. Such impedance stabilization may also reduce the need for bypass capacitors.
The problems outlined above are in large part solved by an interconnecting apparatus employing a lossy power distribution network to reduce power plane resonances. In one embodiment, a printed circuit board includes a lossy power distribution network formed by a pair of parallel planar conductors separated by a dielectric layer. The pair of parallel planar conductors includes a first power supply plane suitable for use, for example, as a ground plane and a second power supply plane suitable for use, for example, as a power plane (e.g., VCC). The dielectric layer has a loss tangent value of at least 0.2, and preferably of at least 0.3. In one embodiment, the dielectric material between the power planes could have a frequency dependent loss tangent, such that a loss tangent value of 0.3 is achieved at and above the lowest resonance frequency of the planes. Due to the relatively large loss tangent characteristic of the dielectric layer separating the power supply planes, the electrical impedance characteristics associated with the power planes may be stabilized, and power plane resonances may be reduced. The printed circuit board may also include one or more signal layers separated from the power planes by respective dielectric layers. The dielectric layers separating the signal layers from the power planes or other signal layers may be associated with much lower loss tangent values, such as in the range of 0-0.05. In this manner, high frequency losses associated with the signal traces may be kept relatively low.
In another embodiment, power plane resonances are suppressed by decreasing the thickness of the dielectric material between the power supply planes to less than 0.5 mils. For example, in one embodiment, the plane separation is preferably reduced to less than 0.2 mils such as, for example, 0.1 mils. In embodiments where the plane separation approaches 0.1 mils or less, plane resonances may be substantially suppressed.
In various embodiments, the power distribution network of a printed circuit board or a semiconductor package interconnect may require relatively large currents. For example, it is not uncommon for systems implemented on printed circuit boards to reach DC current requirements of 100 amps or more. Thus, relatively heavy copper or other conductor layers may be required to handle the large currents. Since a structure that includes very heavy conductive layers on a very thin dielectric layer may be associated with manufacturing and handling problems, a power distribution network may be provided within a printed circuit board or package interconnect in which numerous, relatively thin conductive layers are separated by relatively thin dielectric layers. For example, rather than employing a single pair of relatively thick (e.g., 1-2 mils) conductor layers separated by a relatively thick (e.g., 1-2 mils) dielectric layer in the power distribution network of a printed circuit board, a relatively large number of relatively thin (e.g., 0.05-0.3 mils) dielectric layers with relatively thin (e.g., 0.1-0.2 mils) conductor layers on each side. Alternating conductive layers in the stack up are connected by vias, every second of them connecting to one polarity (e.g., ground) and every other connecting to the other polarity (e.g., VCC). In this manner, the power distribution network may have a relatively low DC resistance to support relatively high currents, while attaining a relatively low high frequency impedance without resonances.
In yet another embodiment, a relatively thin conductive layer is provided between a pair of relatively thick conductive layers. A first relatively thick dielectric layer is provided between one of the thick conductive layers and the thin conductive layer, while a relatively thin dielectric layer is provided between the other relatively thick copper conductive layer and the thin conductive layer. A PCB core constructed according to this embodiment may be associated with relatively good mechanical strength and stability and may be capable of supporting relatively high currents. The structure may further be associated with a relatively low high-frequency impedance without resonances. The thin conductive layer may further be formed in a uniform pattern to create fuses which open if a short occurs through a portion of the thin dielectric.